EP 1 512 254 discloses a method that enables time-controlled messages with good real-time characteristics to be transmitted over an extended ethernet system (hereafter referred to as a TT-(time-triggered) ethernet). TT ethernets distinguish between two message categories: ETE messages, which are conventional event-triggered messages, and TTE messages, which are new, time-triggered messages.
While ETE messages come from a chronologically uncoordinated, open environment and thus can come into time conflict with one another, it is assumed that all TTE messages in a TT ethernet come from a closed TTE network and can be transmitted in a time-controlled manner corresponding to an a priori conflict-free schedule established by a TTE message scheduler without mutual hindrance. The closed TTE network is composed of a number of node computers that communicate via one or more TTE star couplers. Since the closed TTE network can also have a connection to the open world of ETE messages, or ETE messages can be generated within the TTE network, time conflicts can arise between a TTE message originating from a closed environment and an ETE message originating from an open environment. In such a case of conflict, the transmission of the ETE message in a TT ethernet according to EP 1 512 254 will be interrupted by the TTE star coupler so that the TTE message can be transmitted within a guaranteed short transmission time. The TTE star coupler automatically resumes sending the interrupted ETE message after transmission of the TTE message. The TTE star coupler handles all ETE messages according to the ethernet standard (IEEE Ethernet Standard 802.3, URL: http://standards.ieee.org).
ETE message transmission durations that are longer under certain conditions (interruption and resumed sending of an interrupted ETE message by the TTE star coupler) are standard compliant, since the standard contains no specification for transmission duration.
When a TTE message is to be sent, the schedule can be contained either in the TTE message or in the TTE star coupler (or in both). Storage of the schedule in the TTE star coupler entails the following disadvantages:                (i) When the schedule is modified, not only the sending node computer but all affected TTE star couplers must also be modified.        (ii) The schedule stored in a star coupler can be lost if that star coupler suffers a transient malfunction. The star coupler then malfunctions until a new schedule is obtained from an authorized TTE message scheduler.        
This problem does not occur if the schedule information is contained in the TTE message as indicated in EP 1 512 254. However, in this case a defective or malicious node computer can embed a false schedule in the TTE message and thus disrupt all other node computers.